There's no g in Angharad because the the 2nd letter in Angharad is the Welsh letter Ng. Similarly there is no F in Ffion - the first letter is the Welsh letter Ff.
Nia (nee-a) - lovely
Ffion (fee-on) - lovely with possible pronunciation issues
Lowri - lovely with definite pronunciation issues
Seren - nice but also possible pronunciation issues
Avoid Lowri in England, it will be mispronounced to rhyme with Lowry by 90% of the people she meets.
Nia and Ffion are my favourites from your list. You may need to explain - Nee ah, not Nee-uh, and Fee-on, not Fee-un, or 'fuh fie un' as an English person once called my relative! They are very easy to learn though.
Seren is nice but was very popular a few years ago. I think it will be mispronounced as Serrun. Easy to say once you know how tbough, just expect to have to explain the en sound is as in hen, not 'un'.
Alys, Mari and Eira are all favourites of mine. Nia would be on my list too if it wasn't for my love of the boys name Osian - I like more Welsh girls names than boys so Osian is on the list instead of Nia. Beautiful name though.